This application is based on application No. 11-69746 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a metal halide lamp that is used as a headlight or the like.
(2) Description of Related Art
When a metal halide lamp is used as a headlight, the illuminance of the lamp should reach a predetermined level promptly after the lamp is started up so as to ensure the safety. In general, by the passage of a high run-up current through electrodes of the lamp upon start-up, adequate luminous flux is generated to attain the predetermined level of the illuminance in a short period of time after the lamp is started up.
However, the passage of the high run-up current through the electrodes causes an excessive rise in the temperature of the electrodes. As the temperature of the electrodes excessively rises, there may be a possibility that material used for making the electrodes is dispersed. Due to the dispersed material, undesirable effect may be given to the lamp life. One of the subjects in manufacturing metal halide lamps has been increasing their lives.
It is therefore an object of the present invention to provide a long-life metal halide lamp.
The inventor of the present invention conducted analysis from the various viewpoints. As a result of the analysis, the inventor found that the temperature of an electrode can be prevented from excessively rising upon start-up of the lamp by covering the electrode with a metal member in such a manner as to satisfy a certain condition.
The object of the present invention can be achieved by A metal halide lamp made up of: an arc tube made up of a light emitting part and a sealing part at each end of the light emitting part, the light emitting part including a discharge chamber that contains a metal halogen substance; a pair of electrodes that each extend from the sealing part and have inner and outer ends, the inner ends facing each other at a predetermined distance in the discharge chamber so that discharge takes place between the facing inner ends, and the outer ends being sealed in the sealing parts and connected to conductors sealed in the sealing parts; and a pair of metal members that are attached to the pair of electrodes in a one-to-one relationship, each metal member partially covering an electrode within a length measured along the electrode from a boundary between the light emitting part and the sealing part to an inner end of the conductor, wherein an inequality 0.2xe2x89xa6A/Bxe2x89xa61.6 is satisfied, where A is a weight (mg) of the metal member and B is a weight (mg) of a part of the electrode between the boundary and the inner end of the conductor.
With this construction, heat from the electrode is transferred to the metal member, so that the temperature of the electrode can be prevented from excessively rising. Consequently, the lamp life can be increased.